Our recent in vivo studies have suggested that intrapituitary l-thyroxine (T(4)) to 3,5,3'-triiodo-l-thyronine (T(3)) conversion with subsequent nuclear binding of T(3) is an important pathway by which circulating T(4) can inhibit thyrotropin release. The present studies were performed to evaluate various physiological and pharmacological influences on these two processes in rat anterior pituitary tissue. Intact pituitary fragments were incubated in buffer-1% bovine serum albumin containing 0.14 ng/ml [(131)I]T(3) and 3.8 ng/ml [(125)I]T(4). Nuclei were isolated after 3 h of incubation and the bound iodothyronines identified by paper chromatography. There was 0.3-1% [(125)I]T(3) contaminating the medium [(125)I]T(4), and this did not change during incubation. Nuclear [(125)I]T(4) was not decreased by 650-fold excesses of medium T(3) or T(4), suggesting that it was nonspecifically bound. The ratio of nuclear to medium [(131)I]- and [(125)I]T(3) were expressed as nuclear counts per minute per milligram wet weight of tissue:counts per minute per microliter medium. Intrapituitary T(4) to T(3) conversion was evidenced by the fact that the nuclear:medium (N:M) ratio for [(131)I]T(3) was 0.45+/-0.21, whereas that for [(125)I]T(3) was 2.23+/-1.28 (mean+/-SD, n = 51). A ratio (R), the N:M [(125)I]T(3) divided by the N:M [(131)I]T(3), was used as an index of intrapituitary T(4) to T(3) conversion. Increasing medium T(3) concentrations up to 50 ng/ml caused a progressive decrease in the N:M ratio for both T(3) isotopes, but no change in the value for R, indicating that both competed for the same limited-capacity nuclear receptors. Increasing concentrations of medium T(4) caused no change in the N:M [(131)I]T(3) but did cause a significant decrease in R in three of four experiments. These results suggest saturation of T(4)-5'-monodeiodination occurred at lower T(4) concentrations than saturation of nuclear T(3) binding sites. In hypothyroid rats, the N:M ratios for both [(131)I]T(3) and [(125)I]T(3) were increased (P < 0.005), but R was three-fold higher than in controls (P < 0.005). Animals given 10 mug T(4)/100 g body wt per d for 5 d had significantly decreased N:M ratios for both [(131)I]T(3) and [(125)I]T(3), as well as a decreased value for R. In fasted rats, neither N:M ratio was depressed, although hepatic T(4) to T(3) conversion in the same animals was 50% of control (P < 0.005). Iopanoic acid (13 muM), but not 6-n-propylthiouracil (29 muM), decreased the N:M [(125)I]T(3) with a significant decrease in the value for R (P < 0.025 or less). Neither sodium iodide (6 muM) nor thyrotropin-releasing hormone (7-700 nM) affected the T(3) N:M ratios. These results indicate that intrapituitary T(4) to T(3) conversion is stimulated in hypothyroidism and depressed in T(4)-treated animals, whereas opposite changes occur in hepatic T(4)-5'-monodeiodination. Unlike liver, anterior pituitary T(4)-5'-monodeiodination is not affected by fasting or incubation with 6-n-propyl-2-thiouracil, but T(4) to T(3) conversion is inhibited in both by iopanoic acid. These results indicate that there are important differences between anterior pituitary and other tissues in the regulation of T(4)-5'-monodeiodination.